In an Integrated Services Digital Network (ISDN), as defined in the recommendations of the International Telegraph and Telephone Consultative Committee (CCITT), an ISDN telephone subscriber loop includes a U-interface and a T-interface. The U-interface loop includes a two-wire full duplex digital signal transmission line, and extends from a telephone central office or exchange termination (ET) to a network termination (NT). The network termination couples the signals between the U-interface and the T-interface, which includes a four-wire digital signal transmission line for providing transmit and receive signal paths between the network termination (NT) and the terminal equipment (T.sub.E), usually including at least one, and as many as eight, ISDN telephone sets.
The network termination (NT) must be able to correlate the information being received on the receive signal path, from the terminal equipment (TE), with the information which it has just transmitted to the TE on the transmit signal path. In a so-called fixed timing recovery system, the network termination identifies the beginning of the frame being transmitted, delays a predetermined length of time (typically less than a bit period) and then samples the incoming receive path for the corresponding receive signal. This arrangement is not satisfactory, however, because it arbitrarily fixes the maximum loop length to less than the bit period and, where there are several TE's, limits their separation.
It is desirable to have a network termination timing recovery arrangement which is able to accommodate the different configurations envisaged by ISDN T-interface standards, and which will be able to extract the timing despite the different mean phases of the signals received from plural terminal equipments (TE).
Recommended configurations and operating parameters for the T-interface are defined in the CCITT Red Book, layer 1 specification I.430, published 1985, updated 1986, and American National Standard Tl.XYZ.1918Y. (ANSI specification), in both documents specifically at Section 8.6.3. These recommendations entertain provision of up to four kinds of Network Termination to support the various T-interface configurations. Those specified are for Short Passive Bus, Point-to-Point, Extended Passive Bus, and both Point-to-Point and Passive Bus. The round trip delay for Point-to-Point and Extended Passive Bus configurations ranges from 10 to 42 microseconds. For the Short Passive Bus, the round trip delay is in the range of 10 to 14 microseconds, and for the combination of Point-to-Point and Passive Bus, the round trip delay should be in the range of 10 to 13 microseconds for Passive Bus and 10-42 microseconds for Point-to-Point. In the case of the Extended Passive Bus, the differential delay between signals from different termination equipments is in the range 0 to 2 microseconds.
Each Network Terminal (NT) will synchronize its signal transmitted on the transmit path of the T-interface with the signal it is receiving on the U-interface. The difficulty lies in synchronizing the signals being received on the receive path of the T-interface because they will have been originated by different terminal equipments, and so will have different amounts of jitter and phase shift. The problem of adapting timing recovery for different configurations has been addressed by Yasuyuki Okumura, Kazuhiro Hayashi, and Yuji Inoue in a paper entitled "A New Phase Locked Oscillator Adaptable to Input Signals With Periodical Phase Jumps" Proceedings of ISCAS 85,IEEE; by Yasuyuki Okumura, Takashi Yamamoto, and Masasha Kuribayashi, in a paper entitled "Circuit Design and Transmission Performance for ISDN Basic Interface", IEEE, 1986; and by Yasuyuki Okumura and Kazuhiro Hayashi in U.S. Pat. No. 4,682,327 entitled "Polyphase Phase Lock Oscillator", issued July 21, 1987, and incorporated herein by reference. They proposed an adaptive timing extraction method using a polyphase phase-locked oscillator. This oscillator comprises a phase-locked loop for each individual channel which extracts the retiming clock pulse from the channel independently, following segregation of the input signals for each channel by gate signals generated using the marker from the received signal.
This arrangement is not entirely satisfactory because it presumes that each channel will be allocated to a single terminal equipment and moreover requires complex circuitry.
The present invention seeks to ameliorate these problems.
According to one aspect of the present invention, apparatus for transmitting digital signals by way of a transmit path to one or more terminal equipments, and for receiving digital signals by way of a shared receive path from such terminal equipments, said digital signals comprising frames each having a marker comprises:
(i) first means for detecting a marker of a frame transmitted on said transmit path; PA1 (ii) second means operable in response to detection of said marker in the transmitted signal for detecting a corresponding marker of a frame of a digital signal received on said receive path; and PA1 (iii) third means for sampling said received digital signal at a sampling instant determined relative to a specific feature of said received marker.
In preferred embodiments, the aforesaid third means may comprise means for determining the time interval between detection of the transmitted marker, and detection of the received marker and means for variably delaying said sampling instant in dependence upon the duration of such time interval.
The variable delaying means may comprise means for providing a plurality of clock signals phase-displaced relative to one another and to a reference clock, and means for selecting one of said plurality of clock signals for determining said sampling instant. Means for synchronizing the reference signal to said received signal may be provided.
Advantageously, the apparatus may comprise means for detecting the presence or absence of information in the transmitted digital signal. Then the second means may be operative to detect a recurrent feature in the received signal, other than the aforesaid marker, and produce a corresponding signal which can be used in determining the sampling instant. Such corresponding signal would be selected when there was no information, and hence no marker, in the transmitted signal.
The range of variation of the sampling instant, relative to the zero crossing, is conveniently in the range 35-90 percent and preferably less than one half of a bit period.
According to a second aspect of the invention, a method of recovering timing from digital signals received from one or more terminal equipments to which a corresponding digital signal has been transmitted, said digital signals comprising frames each having a marker, comprises the steps of:
(i) detecting a marker of a frame of a digital signal being transmitted to said terminal equipments;
(2) in response to detection of said marker, enabling detection means to detect a corresponding marker in a signal received from one of said terminal equipments; and
(3) sampling said received signal at a sampling instant that is determined relative to a specific feature of said received marker.
The step of sampling the received signal may comprise determining the time interval between detection of the transmitted marker and detection of the received marker and variably delaying said sampling instant in dependence upon said time interval. The variable delaying step may comprise the steps of providing a reference signal synchronized to said received signal, providing a plurality of clock signals phase-displaced relative to one another, and selecting one of said plurality of clock signals for determining said sampling instant. Advantageously, the transmitted signal is monitored for the presence or absence of information and, in addition to detecting the marker, an alternative recurrent feature of the received signal is detected when no information is present in the transmitted signal, and hence no marker, the sample instant is determined using a clock signal derived with reference to the recurrent feature of the received signal.
In both aspects, the marker may comprise a first bit that is a violation of the transmission code, for example, a bipolar violation of an inverted AMI code. In an ISDN signal, the marker may be the frame marker bit and be followed by a second bit of opposite polarity, the second bit serving to maintain dc balance. The aforesaid specific feature of the marker from which the sampling instant is determined may then be the zero crossing between the first and second bits.